During extraction or injection of petroleum oil, natural gas or other fluids into/from subsea reservoirs, there is a possibility of subsidence or elevation of the seabed floor. Monitoring for such motions is desirable both from a production optimization perspective and also from a safety and environmental perspective. Often, relevant water depths are in the range of hundred to several thousand of meters. Measuring minute dimension changes for such depth distances, with sufficient accuracy using remote techniques, is typically considered difficult. Furthermore, since the extent of the reservoirs of oil, natural gas or other fluids can be very extensive in size, multiple measurement location points may be required. The distance between these location points can be considerably large.
One technique that has been used to obtain such minute dimension changes is to employ a remotely operated underwater vehicle (ROV) with recently adjusted and calibrated pressure sensors measuring the water depth at concrete blocks placed on the seabed within an interval for which the sensor drift can be considered negligible. Use of an ROV becomes very expensive as the ROV has to transport a sensor between measurement location points, which are fixed locations, within a short time. The shortness of time is to be sufficiently short to prevent/hinder significant drift in the sensor.
Another technique that has been used to obtain such minute dimension changes is to employ very stable and sensitive pressure sensors placed on the seabed and post process their data. Since the measurement location points may extend beyond the extent of the subsea structures, it is considered expensive to operate these pressure sensors using wired connections. Permanently installed units are thus battery operated and typical communicate using sonic communication.
An additional challenge with using pressure sensors for level monitoring at elevated pressures is that the sensor elements typically need an extended period at elevated pressures before reaching stable performance. These periods have been observed to extend beyond 70 days.
Thus there is a need for continued improvements concerning improved resolution for monitoring small changes from a sampled pressure (e.g., associated with subsea movement such as subsidence).